arch-arm: Initialize some cases of destReg

arch-arm: Initialize some cases of destReg

Some compilers complained that this variable may be uninitialized. This
change initializes it to 0.

Change-Id: I201d75ba05ce49d13bbaf4d67e1c728ef704fdf0
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34335
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: mike upton <michaelupton@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>